In the CDMA (Code Division Multiple Access) method which is employed in a mobile communication system such as the portable telephone system, pilot signals are used as phase reference in performing coherent demodulation. The pilot signals are transmitted by dedicated pilot channels which are different from those for transmitting data of such as audio information.
Also, in other transmission methods and multiplexing methods, etc., reference phase information and data are concurrently transmitted, and the reference phase information is separated by a receiving apparatus to perform coherent reception.
Hereinafter, a receiving apparatus which performs coherent demodulation using reference phase information will be explained with reference to the accompanying drawing.
FIG. 2. shows a block diagram of a receiving apparatus which is adapted for performing coherent reception, which has two coherent receiving units of a coherent receiving unit 201 and a coherent receiving unit 221.
In FIG. 2, the coherent receiving unit 201 includes an antenna 202 for receiving radio waves, an RF (Radio Frequency) receiving unit 203 for receiving signals from the antenna 202, an orthogonal modulator 204 for orthogonal-modulating signals from the RF receiving unit 203, an AD converter 205 for converting analog signals from the orthogonal modulator 204 to digital data, a reference phase inverse spreading unit 206 and a data portion inverse spreading unit 207 which receive output data from the AD converter 205 respectively, a phase correction value calculating unit 208 which receives data from the reference phase inverse spreading unit 206, and a multiplier 209 for multiplying output data or reception data from the data portion inverse spreading unit 207 by output data or phase correction information from the phase correction value calculating unit 208. And, output data from the multiplier 209 is output from the coherent receiving unit 201. Also, the configuration of the coherent receiving unit 221 is similar to that of the coherent receiving unit 201, the explanation of which is omitted.
Output data from the coherent receiving unit 201 and that of the coherent receiving unit 221 are added by an adder 210, and output data from the adder 210 is sent to a frame buffer 211 to be stored. When the amount of output data from the coherent receiving units 201 and 221 stored in the frame buffer 211 comes to a predetermined unit, for example, 1 unit to perform soft decision input error correction, to be described later, data stored in the frame buffer 211 is sent to a multiplier 214. Also, the respective output data from the reference phase inverse spreading unit 206 and the data portion inverse spreading unit 207 of the coherent receiving unit 201 and those from a reference phase inverse spreading unit and a data portion inverse spreading unit of the coherent receiving unit 221 are sent to an SNR calculating unit 212, where the SNR (Signal to Noise Ratio) is calculated. Then, thus calculated SNR is sent to a weighting coefficient calculating unit 213. Weighting coefficients from the weighting coefficient calculating unit 213 are sent to the multiplier 214, where output data from the frame buffer 211 is multiplied by the weighting coefficients. Thus multiplied output data from the multiplier 214 is sent to a bit width restricting unit 216. Also, output data from the SNR calculating unit 212 is sent to a bit width calculating unit 215, where bit width is calculated. Then, output data from the bit width calculating unit 215 is sent to the bit width restricting unit 216, where bit width of output data from the multiplier 214 is restricted. Output data from the bit width restricting unit 216 is sent to a deinterleaver 217 to be deinterleaved, and output data from the deinterleaver 217 is sent to a soft decision input error correcting unit 218.
As described above, output data from the frame buffer 211 is multiplied by the weighting coefficients from the weighting coefficient calculating unit 213, and this processing is intended so as to improve reception characteristics. The following is how the reception characteristics are improved.
For example, it is assumed that data of 1 block unit or the 1 unit is convolutional-encoded and interleaved to be transmitted, and a receiving apparatus of the configuration shown in FIG. 2 receives thus transmitted signals, and then data of 1 block unit is deinterleaved to be viterbi-decoded so as to restore original data. In this case, when taking notice of 1 bit of original data, information of this 1 bit is included in encoded bit of ((confinement length +1)/encoding rate) after convolutional-encoding, and then information of this 1 bit is scattered over 1 block for transmission and reception after interleaving. When 1 block for transmission and reception is divided into N divisional blocks each of equal size and SNR (Signal to Noise Ratio) is calculated, error of data in divisional blocks of high SNR is small since transmission state thereof is desirable, while error of data in divisional blocks of low SNR is large since transmission state thereof is undesirable. Based on this information, by multiplying reception intensity values of reception data by weighting values calculated for each divisional block of a block, using large weighting values for high SNR and small weighting values for low SNR, and deinterleaving data of the block so as to restore original data, influence of data with large error can be reduced. That is, it becomes possible to improve bit error rate after viterbi-decoding by using data with small error.
Conventionally, in performing coherent reception, reception data is multiplied by phase correction information, and resulting data undergoes buffering under a frame buffer. Then, weighting coefficients for performing soft decision input error correction are calculated using values based on SNR of reception data, and then output data from the frame buffer is multiplied by the weighting coefficients at the upstream stage of a soft decision input error correcting unit.
So, in this conventional manner, two times of multiplication, that is multiplication for phase correction and that for weighting, is required for one of data.
Furthermore, in performing buffering for data which has undergone multiplication for phase correction, since bit width of data is caused to be enlarged due to wide dynamic range, downstream circuits of a multiplier and a RAM are caused to be enlarged in size.
Specifically, in FIG. 2, each of the two coherent receiving units, or the coherent receiving unit 201 and the coherent receiving unit 221, performs two times of multiplication, that is, multiplication for phase correction at the multiplier 209 and multiplication for weighting at the multiplier 214, and circuits of the adder 210 and the frame buffer 211 are caused to be enlarged in size due to enlarged bit width.